Toasting oak by convection: the idea came from a bakery
How we went from a rack of croissants to an oenological wood oven, and what this way of toasting really solves.
The story is true and we like telling it, because it says something about the way we work. Back in the 90s, Stéphane was waiting in a bakery. The baker pulled his croissants out of a convection oven: they were baked the same way, at the top of the rack and at the bottom. At the time we were toasting oak with cooperage methods, and we knew our main problem was right there, in the gap between two pieces from the same batch. We went looking for the know how where it already existed, with engineers who came from pastry machinery.
What goes wrong with traditional toasting?
A flame or infrared radiation heats the surface first. The heat then moves toward the core by conduction, and a temperature gradient sets in between the two. The thicker the piece, the more pronounced that gradient is.
On a chip of 2 to 4 mm, the gap stays small and the problem is minor. On an 18 mm stave or a 22 mm insert stick, it is not. The reactions that create aromas, lignin degradation in particular, do not happen the same way 2 mm below the surface and at the center. You end up with a piece whose profile depends on where you taste it, and two pieces from the same batch do not necessarily resemble each other.
What convection changes
Our oven circulates hot air around each piece, with controlled temperature and air speed. The transfer happens by forced convection, which reaches the wood from every side instead of cooking it from one face.
What we observe, measured on our own batches: the temperature gap between the surface and the core narrows clearly, and the spread between two pieces from the same batch goes down. We are not saying the temperature is identical at every point in the wood, that would be false. We are saying the gap has become small enough that we can name a profile and find it again from one batch to the next.
| Format | Thickness | Toasting challenge |
|---|---|---|
| Fine chips | 2 to 4 mm | low gradient, toasting is the main lever for the profile |
| Staves | 7 mm | noticeable gradient, to watch over the length of the cycle |
| Staves | 18 mm | decisive gradient, this is the format that motivated the oven |
| Barrel inserts, sticks | 22 mm | same constraint as the 18 mm stave, on a thicker piece |
Is that enough to guarantee a profile?
No. Toasting is one parameter among others. Before it comes the wood: our oak is air dried for 36 months, and two lots of stave wood from different forests do not reach the oven with the same composition. We correct part of that variability through sorting and through cycle settings, we do not correct all of it, and we keep looking on that side.
There is also everything that happens after us. The same stave does not give the same wine depending on the tank, the cellar temperature and the contact time. What we measure well in the lab is measured poorly in a 300 hL tank in October, and that goes for toasting too.
Why we talk about it
Because it is what makes the rest possible: named profiles, Primary Colors to blend yourself or blends already made in La Recette. Without steady toasting, a profile name does not mean much. Today we offer this toasting work to fellow coopers, on their own wood.
If you want to see what these profiles do on your volumes, everything is in the wood shop, and the questions that come up most often are gathered in our questions and answers. If one of our numbers does not hold up at your place, tell us: we would rather correct it than defend it.
Read next : Chips, staves or barrel inserts: how to choose · Le Nuancier des Possibles : les "Couleurs Primaires"